Automatic telephone trunking system



Nov. 14, 1933. R. s. RICHARDSON 1,934,737

AUTOMATIC TELEPHONE TRUNKING SYSTEM Filed Oct. 12, 1931 2 Sheets-Sheet 1 y .u\ 9h Wk 5 M m um wEnQ w w n E H E QNQM v2 1 Q E E H OFF/CE E E a. 7E F W MM M 4 ll I I I I .lllwlllllw lllllll E T A 2) Q3 M I A 6 M X 2 Rodney 5-. Richardson "Ill "ll 2 sheets-sheet 2 R. G. RICHARDSON AUTOMATIC TELEPHONE TRUNKING SYSTEM Filed Oct. 12, 1931 "3 Q lllllll ll Nov. 14, 1933.

i: llllll I-LWEEIH HUCII'IEH S: Richardson Patented Nov. 14, I933 PATENT orirrcs 1,934,737 g I AUTOMATIC TELEPHONE TRUNIHNG SYSTEM.

Rodney GrRich'ardson, Chicago, Ill; assignor, by mesne assignments, to Associated Electric Laboratories, Ina, Chicago, lit,- a corporation of Delaware Application'October 12, 1931. Serial. No. 568,349

Claims. (01.179 16 Thisinvention relates to automatic telephone trunking systems, and the object of the invention is to provide new and improved trunking ar rangements tending to greater efiiciency and 5 economy of apparatus than is found in systems heretofore available. I

The invention contemplates the use of socalled drop-back or digit-absorbing selectors, such as are shown in the patent to Martin, No. 1,640,551, the patent to Bellamy No. 1,694,615, and the patent to Suekoii, No. 1,531,244. Trunking systems using such drop-back selectors are shown in the patents mentioned. The patent to Lomax, No. 1,747,478, also shows a trunking system using drop-back selectors. These Known trunking systems, however, are applicable only to certain particular situations and they afford no solution to problems arising in many other situations. According to the present invention, the use of drop-back selectors is greatlyv extended. These comparitively'inexpensive switches are used in new and, unexpected ways to introduce plant economies and to solve trunking problems which heretofore could not be taken care of without the use of much more expensive equipment; 1

The invention is illustrated in the accompanying drawings in which Fig. 1 is a schematic diagram showing the trunk lines interconnecting the oflices or exchanges in a multi-ofiice automatic telephone'system; Fig. 2 is a more detailed trunking diagram which shows the trunking in the ofiice A and in the tandem omce E of Fig. 1; while Fig. 3 is a detailed trunking diagram which'shows a modified form of the invention applied to the individual offices of a two-oifice system.

Referring to Fig. 1, the system therein depicted comprises a tandem ofiice E and a number of outlying offices A, B, C, D, and F. It may be assumed that these oifices are all standard 10,000-line ofizlces, at least the full quota of 10,000 subscrib ers numbers is allotted to each office. A uniform six-digit numbering system is used, that is, each The subscribers number comprises six digits. first two digits in each number relate to the interofilee trunking, while the last four digits are the so-called local or numerical digits which relate to the local trunking. If desired, of course,

of these latter 'ofiices.

- The interofiice trunking features the use of drop-back selectors, according to a systematic plan which has been worked out, providing for tandem trunking while preserving the'uniform numbering system. As shown in Fig. 1, the tandem office E is connected with each of the other ohices by direct trunk lines. As illustrated, it is assumed that these are one-way trunk lines, although, of course, two-way trunks could be used. There are, therefore, two groups of trunk lines extending between the tandem ofiice E and each of the'other oinces. One group is for handling traffic from the tandem ofiice to the particular oflice in question, while the other group handles traffic fromsuch office to the tandem oiiice. There are also groups of direct trunks connecting certain of the outlying oilices, as for instance offices A and B, offices B and'C, etc. Not all of the outlying oiiices are thus connected, however. Calls from omce A to office F, for instance, are routed via the tandem office E.

All this is clearly shown in Fig. 1, in which each group of trunk lines is indicated by a single line drawn between the circles representing oifices. Fig. 1 also shows the office digits assigned to each ofiice, and, in connection with the groups of trunk lines, it shows the digits which are dialled to give access to each'group. The office designation assigned to ofiice A, for instance, is 23, while the office designation assigned to the tandem ofiice E is 65. The group of trunk lines extending from ofice E to ofiice A bears the number 23, which indicatesthat the digits 2 and 3 must be dialled to reach a trunk in this group. The group of trunk'lines extending from oflice A to oilice E bears the designation 65-4-7, which indicates that a trunk in this group may be reached by dialling the digit 6, the digit 5, the digit 4, or the digit '7. The point here is that this groupof trunk lines is used for handling traffic from ofi'ice A to ofiices E, C, D, and F, and is reached by dialling the first digit of the oflice designation of any The numbering of the other groups of trunk lines can readily be understood from the foregoing.

Reference may now be had also to Fig. 2 for a moredetailed explanation. That portion of Fig. 2 to the leftof the dotted line indicates equipment in oflice Afwhile the portion to the right indicates equipment in the tandem ofiice E.

At the upper left-hand corner of Fig. 2 a subscribers station is indicated, the telephone number of which is 23XXXX. The last four digits, being immaterial,; are each represented by X. This line has access by way of any Well-known switching device, such as the rotary lineswitch RS, to a group of firstselectors, oneof which is FS.

On the second level of its bank the first selector F8, in common with other similar selectors, has access to trunk lines extending to local thousands selectors such as MS. The selectors such as MS are of the drop-back type, similar to the selectors shown in the Bellamy patent or the Suekoff patent, and they always cancel the first digit received after being seized. The letters (DB) inserted in the rectangle which represents the selector MS in the drawings indicate that the selector is of the drop-back type and similar labels serve to identify the otherselectors of this type.

On the third level of their banks the first selectors such as FS have access to a group of trunk lines 3, Fig. l and Fig. 2, which extend to ofiice B, where they terminate in thousands selectors similar tothe thousands selectors MS or MSZ. The latter selectors terminate in trunk lines incoming to office A from ofiice B, and the arrangement in ofiiceB is the same as regards the trunk lines incoming from oil-loo A.

The fourth, fifth, sixth, and seventh levels of the selectors such as F8 are multipled together and give access to a group of trunk lines 6547, extending to the tandem ofiice E, where they terminate in ofiice selectors suchas OS. These office selectors are standard Strowger switches and do not cancel any digits.

Referring now to oifice E, there is showna subscriber-s station having the telephone number XXXX. This line has access through a rotary lin'eswitch R52 to a group of first selectors such as F82. These first selectors are of the dropback variety previously mentioned and always cancel the first digit. The banks of the first selectors F52 are connected in multiple with the banks of the oilice selectors. 3

The ofiice selectors comprise, in addition to the ofiice selectors OS previously'mentioned, a group including 0S2, a group including 0S3,a group including CS4, and a group including 035. These groups or" office selectors terminate groups of trunk lines incoming from ofiices B, C, D, and F, respectively. he banks of all the oifice selectors are multipled together, and with the banks of the first selectors PS2, as previouslystated.

On the third level of their banks the cities selectors OS and the first selectors PS2 have access to a group of trunk lines 23 extending to ofiice.

A, where they terminate in a group of thousands selectors M553. On the second, fourth, sixth,and eighth levels these ofiice selectors have access to groups of trunk lines extending to offices C, B, F, and D, respectively. These latter groups of trunk lines terminate in groups or thousands selectors similar tel/5S3. 7

On the'fifth level the oiiice selectors and the first selectors in ofjce E have access to agroup of trunk lines extending to the local thousands se-' lector such as M84. These selectors are standard selectors and do not cancel any digits.

The trunking beyond the thousands selectors is not shown as it is well known. It may be stated, however, that the thousands selectors MS4 in ofiice E have access on the various levels of their banks to groups of hundreds selectors, and these latter in turn to groups of connectors, which have access to the subscribers lines.

In oifice A also the thousands selectors, such asMS', M82, and M33, have their banks rnultipled together and give access to trunk lines extending to groups of hundreds selectors, which have access to groups of connectors. The same is true of the other ofoces.

Although statements are made herein without qualification to the effect that banks are multipled, this may be only partially true in certain cases where large trunk groups are required. It will be understood that the multiple may be sectionalized as desired according to well known principles of grading, or the different groups of switches whose banks are stated to be multipled may have access on certain levels to separate groups of trunk lines leading to secondary lineswitches. These facts do not alter the principles involved, and for simplicity, therefore, the banks of switches of the same order are shown as being irectly multiplied together.

It will'be convenient now to explain the establishment of a few typical connections. It will be assumed first that the subscriber at station ZSXXXX desires to obtain connection with another subscriber in the same office whose tele phone number also is ZBXXXX di -ering, of course, in one or more of the last four digits. On removing the receiver, the lineswitch RS operates to select an idle first selector such as PS. The first digit 2 of the ofiice designation is then dialled, operating the selector F5 to the second level, whereupon it rotates in and selectsan idle thous :ls selector such as MS. The second office digit 8 is then dialled and is cancelled by the selector MS, the circuits of the selector being "automatically altered at the same time so that the next digit will not be cancelled. The four local digits aredialled next, the first digit operating the selector M to selectan idle hundreds selector in the desired thousand, the second operating the selected hundreds selector to select a connector in the desired hundred, and the third and fourth local digits operating the selected conhector to select and connect with the desired subscribers line. The switches are released in the usual manner, when the calling subscriber hangs up the receiver.

It will be assumed now that the subscriber at station ZSXXXX desires to extend a call to the subscriber at station 34=XXXX in office B. On removing the receiver, the lineswitch RS operates and selects an idlefirst select r such as FS. When the first ofice-designating digit 3 is dialled, the first selector FS is raised to the third level and rotates in to select an idle trunk in the group 3,. extending to the office B, where these trunks terminate in thousands selectors sirn ilar to MS". When the next digit, 4, of the ofiice designation is dialled, the di it is cancelled by the thousands selector in use. When the next digit of the nurnber is dialled the thousands selector isoperated to select an idle hundreds selector, the latter then responds to the fourth digit the number to select an idle connector in the sired group. The last two digits then operate e connector to complete the connection.

Suppose now the subscriber at station LBXXXX desires to obtain connection with a station, such as G-EXXXX in ofiice E. On removing the receiver to initiate the call the line-switch RS operates as before to select an idle first selector, for example, the selector On dialling the first office digit 6, the selector FS' is raised to the sixth level, and then selects a trunk leading to an idle office selector such as OS, in ofiice E. The ofiioe selector OS responds to the second oflice digit 5 and is raised to the fifth level, thereupon selecting an idle local thousands selector such as M84. The four localdigits now-control the-operation of the thousands selectorM84, a hundreds selector, and a connector to complete the desired connection. 1

It may be assumed now that a subscriber at station 65XXXX desires toobtain a connection with another subscriber in the same ofiice, that is, in'the tandem office, whose number will be 65XXXX. -Upon removing the receiver, the

lineswitch R82 is operated in the usual manner to select a trunk line leading to an idle first selector, such as F82. The first digit (6) of the ofiice designation is now dialled, and this digit is cancelled by the first selector F82, the selector at the same time alteringits circuit so that the next digit will not be cancelled. The operation of establishing a local connection in the tandem office, therefore, differs from the operation of establishing a local connection in an outlying ofiice, such as the ofiice A. In thetandem office, the first digit of the ofiice designation is cancelled, whereas in the outlying ofiice the first digit of the office designation is effective and the second digit is cancelled. Continuing with the operation, when the second digit (5) of the ofiice designation is dialled, the selector F82 is raised tothe fifth level, and thereupon selects a trunk line leading to an idle thousands selector, such as M84, it being understood that the banks of the selector, such as F82, are multipled' with the banks of the office selectors. The four local digits of the niunber are now dialled and are effective to operate the thousands selector M84, a hundreds selector, and a connector to select and complete the connection to the desired subscribers line.

Suppose now that a subscriber at station 65XXXX desires to obtain connection with a subscriber in office A, for instance the subscriber at station ZSXXXX. When the receiver isremoved the lineswitch RS2 operates as usual to selectan' idle first selector, such, as F82. The first ofiice digit 2 being dialled, this digit is cancelled by the selector F82. When the second office digit 3 is dialled, the selector F82 is raised to the third level, and selects an idle trunk line in group 23, leading to a thousands selector, such as M83, in office A. The next four digits of the number,'the local digits, operate the thousands selector M83, a hundreds selector, and a connector to complete the desired connection. m

It will be seen from the foregoing that the establishment ofa call from a station, such as 23XXXX in an outlying office to a station, such as SSXXXX, in the tandem office; differs from the establishment of a connection in the reverse direction. In setting up a call from a station in ofiice A to a station in office E,'a1l the digits-are eifective, while in setting up a call from a station in office E to astation in office A the first ofiice digit is cancelled. a

Considering now the establishment of a tandem connection through ofiice E, it may be assumed that the subscriber at station 23XXXX in oflice 'A desires to obtain connection with' a subscriber whose number is 'lfiXXX X in oifice F.

On removing the receiver, lineswitch RS1 operates in the usual manner to select an idle first selector, such as F81, When the first ofiice digit 7 is dialled, the selector F81 is raised to the seventh level, andthereupon selects an idle trunk line inthe group 6 5 -47, leading to an-ofiice selector suchasOSl in the tandem ofiice E. The second ofiice digit 6 may now be dia1led,resulting in operating the office selector 081 to the tion, therefore, will makeit clear how the various types of calls involving the other offices are'established. Local connections in ofices B, C, D, and F, for instance, are established in the same manner as are calls in office A. Calls. from an ofiice, such as B, to an ofiice, such as C, involving the use of direct trunks between outlying offices, are established in the same way as calls between office A and ofiice B. Tandem calls from ofiices,

such as D and F, to offices, such as A or B, are 1 established in the same Way as the hereinbeforedescribed tandem call from. oflice A. to office F. It will not be necessary, therefore, to describe the calls in detail.

It will be seen that an economical trunking layout has been devised applicable especially to moderate sizedmulti-ofiice networks, and possessing a number of advantages. Although both tandem and direct trunking are used, all calls are set up in accordance with a uniform numbering system. The drop-back selectors used are of the simplest type, as they always cancel the first digit only and are not required to discriminate as to the value of the digit. The number of switches. involved in any call is reduced to a minimum. Only on tandem calls arethe full complement of five switches required and on all other calls only four switches are used.

Referring now to Fig. 3 this drawing shows an automatic telephone. system comprising two offices. Each office comprises two groups or thousands. of subscribers lines. The office to the left of the dotted line will be referred to as the No. l ofiice, while the ofiice to the right of the dotted line will be referred to as theNo. 2 ofiice. A uniform five-digit numbering system is used, that is, each subscribers telephone number comprises five digits. The first digit in each number, is the office designating digit. A particular feature of the invention is the use of one of the local groups or thousands in each office as a tandem switching point on calls incoming from the other office.

Referring to office No. l, as mentioned hereinbefore, this ofiice comprises. two main. groups of subscribers lines. ferred to as the third thousand and comprises 600 subscribers lines. Each subscribers line, such as the line of station 13520, has individual thereto a rotary lineswitch such as R83, which has access to a group of combination first, second, i

RS5, which has access to a group of combina- 1 tion first, second, and third selectors, such as C82.

The combination first, second, and third selectors, such as C81 and C82, are of. the dropbacktype and are-of the general type of switch which is shown inthe Lomax patenthereim.

The first group may be reits a group of tandem selectors, such as TS2.

before referred-to; The combination selectors, such as CS1 and CS2, however, are arranged to drop back on two levels instead of one. The drop-back selector shown in Fig. 2 of Lomax, therefore, is modified to t e extent of having a second set of shaft springs like the set there shown. The springs 9 of the second set are connected in multiple to springs 9 of the first set, while springs 10 of the second set are connected in series with springs 15 of the first set.

The sixnpper levels of the combination selectors, such as CS1, are connected in multiple and give access to six groups of connectors, such as connector Cl, which is reached by way of the fifth level. The connector C1 and the other connectors of this group have access to subscribers lines in the fifth hundred of the third thousand.

Theupper six levels of the combination selectors CS2 are also connected in multiple and give access to six groups of connectors, including the connector C2, reached by way of the fifth level. The connectors of the group including connector C2 have access to the fifth ,hundred in the fourth thousand. The third level of the selector CS2 is also multipled, as indicated in the drawings. For convenience, the bracket which indicates the multiple connections includes the fourth level, but it is immaterial whether this level be multipled or not, unless the fourth level is to be used for trunking to connectors, as will be explained subsequently.

The fourth level of the combination selectors, such as CS1 of the third thousand, gives access to trunk lines which terminate in hundreds selectors, such as I-ISZ, in the fourth thousand. The banks of these hundred selectors are multipled with the banks of the combination selectors such as CS2.

The third level of thecombinaticn selectors CS2 of the fourth thousand gives access to trunk lines leading to hundreds selectors, such as H81, in the third thousand. The banks of the hundreds selector HSl are connected in multiple with the banks of the combination selectors, such as CS1.

The No. 2 oihce comprises two main groups, the same as the No. 1 office, and in general is laid out like the No. 1 oiiice. It will not be necessary, therefore, to go over the trunking in detail.

The interoflice trunking may now be referred to. The second level of the combination selectors, such CS1, in the No. 1 office, give access to a group of trunk lines which leadto the No. 2 ofiice, and they terminate in a group of tandem selectors, such as TS3. These tandem selectors have their banks connected in multiple with the banks of the'combination selectors CS3 and the banks of the hundreds selectors BS2. The second level of the combination selectors CS2 in the No. 1 ofiice give access to agroup of trunk lines leading to the No. 2 office, and there terminatein Tandem selectors TS2 have their banks connected in multiple with the combination selectors CS4 and the hundreds selector KS4. The foregoing arrangement is indicated in the drawing. It may be preferred, however, and ordinarily the practice would be, to have a certain number of first-choice trunks accessible fronrthe combination selector CS1 and leading to tandem selectors such as TS3, a corresponding number of first-choice trunks accessible to the combination selectors CS2 and leading to the tandem selectors TS2, and a group such as CS1. i .tor CS1 operates to cancel the digit. It will be of over-flow trunks accessible in common to the combination selectors CS1 andCSZ and leading to tandem selectors which are distributed between the third and fourth thousandsiof the No. 2 ofiice.

The trunking from the No. 2 oince to the No.

1 omce may be the same as has been described for trunking from the No. 1 office to the No. 2 ofiice. However, a somewhat simpler arrange-. ment maybe adopted in case it is unnecessary to take advantage of all the features of the present invention. As shown in the drawings, the first level of the combination selectors CS3 in the No. 2 office is multipled with the first level of the combination selectors CS4, and from these multiply connected levels a single group of trunk lines extends to the tandem selectors, such as TSl in the No. 1 office. The banks of the tandem selectors, such as TSl, are connected in multiple with the banks of the combination selectors CS2 and the hundreds selector KS2, at least the multiple extends over all those levels which are used for trunking to connector switches and to the hundreds selector HSl.

V The levels at which the various drop-back selectors cancel digits is determined by the numbering. In the present layout, the combination selectors, suchas CS1, cancel the digits land 3, and are, therefore, arranged to drop back on these levels. The combination selectors such as CS2 cancel digits 1 and 4. and are arranged to drop back on the corresponding levels. Combination selectors CS3 and CS4 are the same as combination selectors CS1 and CS2, respectively, except that they cancel the digit 2 instead of the digit 1. Tandem selectors TSl are arranged to cancel the fourth digit, and these selectors, therefore, drop back on this level. The tandem selectors TS2 are similar to the tandem selectors TSl. The tandem selectors TS3 are the same, except that they-are arranged to cancel the third digit instead of the fourth. Appropriate labels on the drawings-indicate the levels on which the various selectorsdrop back.

It may be mentioned that the tandem selectors described hereinmay be of the type disclosed in-the Lomax patent hereinbefore referred to. v

'It may now be assumed thatthe subscriber at station 13520 desires to extend a call to station 13510, in other words, to a station in' the same main group in the same oihce. Upon removing the receiver, .the lineswitch RS3 operates and extends the line to an idle combination selector On dialling the di it L the selecunderstood that the selector is raised to the first level and then releases. The next digit is the digit 3, and this-digit is cancelled also. Thus the combination selector CS1 is in normal position 1 after the dialling of the first two digits. The next digitdialled is the digit 5, which raises the combination selector CS1 to the fifth level,

whereupon the selector rotates in and selects a trunk line leading to an idle connector, such as C1.) The next two digits, 1 and 0, operate the connector C1 to complete the desired connection. The switches release in the usual manner when the receiver is hung up at station 13520.

It may be assumed now that the subscriber at station 13520 desires to obtain a'connection'with the subscriber at station 14520, that is, to a subscriber in the other main group in the same office. Upon lifting the receiver, the rotary lineswitch RS3 operates to findan idle combination selector such as CS1. The first digit dialled is the digit 1, which is cancelled as before described. The second digit dialled is the digit 4, which raises the combination selector CS1 to the fourth level. Hunting in this level in the usual manner, the combination selector CS1 selects an idle trunk line leading to a hundreds selector, such as 1-132. The next digit dialled is the digit 5, which operates the hundreds selector HS2 to therfifth level, on which level the selector rotates in order to find an idle connector, such as connector C2. The digits 2 and 0 operate the connector C2 to complete the connection to the desired station, 14520.

A call from station 14520 to station 13520 follows the same principle as the call just above I described. Selector CS2 cancels the first digit 1, the second digit 3 is used to raise the selector CS2 to the third level and select a hundreds selector such as H551, the third digit 5 operates the hundreds selector I-ISl to select an idle connector, while the last two digits operate the connector, such as C1, to complete the desired connection. Calls from station 14520 to another station within the same main group, that is, within the four thousand group, are established in the same way as was described in the case of station 13520 calling station 13510.

It will now be assumed that the subscriber at station 13520 desires to obtain a connection with the subscriber at station 24550,.that is, to a subscriber in the fourth thousand of the No. 2 ofiice. On removing the receiver, the rotary lineswitch RS3 operates in. the usual manner to select an idle combination selector such as CS1. On dialling the first digit 2, the office designating digit, the combination selector CS1 is raised to the second level, whereupon it selects an idle trunk line leading to the No. 2 office, and there terminating in a tandem selector, such as TS3. The next digit dialled is the digit 4, which operates the tandem selector TS2 to the fourth level. On this level the tandem selector hunts for an idle trunk leading to at hundreds selector, such as HS 4. The next digit dialled is the digit 5, which operates the hundreds selector 1-184 .to the fifth level, whereupon the hundreds selector selects a trunk line leading to an idle connector, such as C4. The last two digits, 5 and 0, operate the connector C4 to complete the desired connection to station 24550.

It may be assumed now that the subscriber at station 13520 desires to extend a connection to station 23530 in the third .thousand group of the No. 2 office. On removing the receiver, the lineswitch RS3 operates as usual, selecting an idle combination selector, such as CS1. Responsive to the first digit 2, the combination selector CS1 selects a trunk leading to the No. 2 ofiice and there terminating in a tandem selector, such as RS3. The next digit dialled is the digit 3, which is cancelled by the tandem selector TS3. The

next digit dialled is the digit 5, which operates the tandem selector TS3 to the fifth level, on which level it selects an idle trunk leading to a connector, such as C3. Connector C3 responds to the last two digits, 3 and 0, and completes the desired connection to station 23530.

The foregoing calls are typical, and the explana tion given will enable any one to readily see how all of the other possible calls are set up. 1

As regards calls made within the No. 1 office, it will be observed that some of these calls are completed by means of only two switches, while calling stations.

other c'alls require three switches for their completion. Thus, a call from station 13520 to station 13510 requires only a selector, such as CS1, and a connector, such as C1. The same is true of a call from station 14520 to another station in the four thousand group. Intergroup calls,

however, require three switches for their completion. For instance, a call from station 13520 to station 14520 requires a combination selector, such as CS1, a hundreds selector such as H52, and a connector C2 for its completion. It will be obvious that under ordinary circumstances about half the calls local to the No. 1 ofiice can be completed in this system by means of only two switches.

The foregoing leads up to certain provisions which can be availed of in order to insure that the possible economies are secured, or to increase the normal economies which are inherent in the system. It Will be clear that the saving of one switch on fifty percent. of the local calls will only obtain if fifty percent. of such calls initiated in the No. 1 oifice, for instance, are for stations which are in the same group as the this saving, the best plan to follow is to give the lineswitches, such as RS3 and RS5, equal access to the selectors CS1 and CS2 in both thousands. This will insure that approximately half the calls from any calling line go to combination selectors, such as CS1, in the third thousand, while the other half go to combination selectors, such as CS2, in the fourth thousand, and will insure that approximately half the time the calling and called stations will be in the same main group.

It is possible, however, to obtain a considerably greater economy by putting in efiect a routine of checking the calls after the exchange is installed so as to determine, in the case of busy calling lines, which stations are the most frequently called. In the case of station 13510, for instance, service observations may indicate that a very large percentage of the calls from this station go to a called station in the fourth thousand group. This being the'case, an economy of switches can be effected ifthe lineswitch assigned to station 13510 has access to combination selectors, such as CS2 in the fourth thousand group rather than to combination selectors, such as CS1, in the third thousand group. The shifting of the trafiic from one group of combination selectors to the other is very readily taken care of by means of an intermediate distributing frame. The drawings indicate in the upper left-hand corner a main distributing frame labeled M. D. F., and an intermediate distributing frame labeled I. D. F. According to known practice, the outside lines terminate on one side of the main distributing frame, and from the other side of the main distributing frame cables extend to one side of the intermediate distributing frame. At this "eds point, also, are terminated cables coming from the connector bank multiples. On the'other side of the intermediate distributing frame cables run to the lineswitchesp It will readily'be seen now that any line, such as the line of station 13510, can be cross-connected to a lineswitch which has access to combination selectors such asCS3 in the third thousand or to a lineswitch, such as RS4, which has access to combination selectors, such as CS2, in the fourth thousand. It may be assumed that station 13510, from the telephone number, was originally crosseconnected to a lineswitch having access to combination selector CS1,

but the drawing indicates that the cross-connece 1 tion has been changed to lineswitch RS4 which has access to combination selectors in the fourth thousand.

It will readily be seen that considerable may be accomplished along the plan outlined, with a view to getting calling and called subscribers in the same main group as far as possible, with the idea of using only two switches for the maximum number of calls. Where an automatic exchange, following this plan, is to be installed in place of a manual exchange, possibly sufficient data on the parties customarily called by each subscriber can be obtained beforehand so as to effect a considerable saving at the start. In other cases, trafiic observation which takes into account not only the calling party but the party called will indicate how the cross-connections to lineswitches may be changed to advantage for the purpose of obviating the necessity. of installing additional exchange equipment as the traific grows.

As regards the tandem trunking between ofiices, it will be observed that the feature involved is to use one of the thousands as a tandem switching point, which enables one switch to be saved on approximately fifty percent. of the tandem calls.

With the arrangement of trunking, as shown in the drawings, for calling from the No. 1 exchange to the No. 2 exchange, considerably more than fifty percent. of the calls may be included in the group which requires less switches than ordinarily. It will be observed that the combination selectors CS1 have access to tandem selectors in the third main group of the No. 2 oflice, while the combination selectors CS2 have access to trunks extending to the tandem selectors in the fourth main group or" the No. 2 ofilce. At least the first-choice trunks accessible to the combination selectors CS1 and CS2 are assumed to be connected in this manner. It follows that if a subscriber in the No. 1 ofiice, such as the subscriber at station 13520, confines his interofiice calls, to a large extent, to one or two particular subscribers in the No. 2 office, these subscribers should, if possible, be terminated in the third thousand group of the No. 2 ofiice, or if this is impossible the line leading from station 13520 should be changed over to a lineswitch which has access to the combination selector CS2. The first condition can usually be met where the particulars of the calling are known beforehand, and care can be taken to assign the telephone numbers so as to bring about the desired results. In the case where this is not done, however, or where the calling habits change, the desired condition can best be brought about by changing to different lineswitches, as mentioned, for it is generally considered undesirable to change a subscribers telephone number. Of course, care must be taken that a change in the intermediate frame jumpering to provide economies on interoffice calls does not interfere with economies on calls within the same exchange, but, if a record is kept of both local and interoffice calls made by particular lines, the best cross-connecting to give the maximum economies in both cases can readily be worked out.

Something can be done even with the trunking shown fol-handling calls from the No. 2 office to the No. 1 ofiice in order to, affect a saving of switches. A survey will generally show that a number of stations in the No. 2 office habitually direct a large proportion of their calls to particular stations in the other ofiice. This survey will then give a small group of stations,'which, as

called lines should be assigned tothe main group in which the incoming tandem selectors are lo,- cated, and thus avoid the necessity of using a separate hundreds selector on calls to these stations.

For the sake of simplicity, the system has been described as comprising two oifices, each ofiice comprising two main groups of 606 lines each. It will be appreciated that if only 600 lines are used in each main group, at least two levels of the combination selectors, such as CS1, i are wasted. The second level is used for interoffice trunking, while the fourth level is used for intergroup trunking, but no use is made of the first and third levels. It will readily be appreciated now that, by following the principles set forth in the patent'to Martin or the patent to Bellamy hereinbeiore referred to, the combination selectors can each be provided with a relay which will open the release magnet circuit after the second digit has been cancelled. This will prevent the combination selectors from cancelling the digits 1 and 3 if they are dialled the second time, and will enable these two levels to be used for trunking to connector switches. Thus, the size of each of the main groups can be increased to 800 lines if desired.

What is claimed is:

1. In a telephone system, a tandem ofiice, a plurality of non-tandem offices, subscribers lines connected to said oifices all having the same number of digits in their telephone numbers, means including a certain number of automatic switches for completing a tandem call, means including a lesser number of switches for completing a nontandem call, and means in a certain switch which is used exclusively for non-tandem calls for cancelling a digit.

2. In a telephone system, a tandem office, a plurality of non-tandem offices, subscribers lines connected to said ofiices all having the same number of digits in their telephone numbers, means including a certain number of switches for com pleting a tandem call, means including a lesser number of switches for completing non-tandem calls, and means for cancelling the first or second digit on any non-tandem call depending on whether the call originates in the tandem office or in a non-tandem office.

3. In a telephone system, two offices, non-dropback first selectors in the first office, drop-back first selectors in the second oifice, drop-back local selectors in the first office accessible to the first selectors therein, and non-drop-back local selectors in the first office of the same order as the first mentioned local selectors accessible over trunk lines to the said first selectors in the second ofiice.

4. In a telephone system, an oflice, a series of switches in said office for setting up local connections, a first switch in said series responsive to the first digit of a subscribers number to select the second switch of the series, means in the second switch for cancelling the second digit of the number, said second switch responsive to the third digit of the number, and other switches in said series responsive to the remaining digits.

5. In a telephone system, a plurality of ofi'ices, a tandem office, drop-back first selectors in said tandem ofiice, non-drop-back first selectors in said ofiices, trunks accessible to said non-dropback selectors extending to said tandem ofiice, non-drop-back ofiice selectors terminating said trunks, and multiple connections between the banks of said drop-back selectors and said ofiice selectors.

6. Ina telephone system, a tandem ofiice, a non-tandem oifice, a second non-tandem office, trunks extending from said first non-tandem ofiice to said second non-tandem office, drop-back selectors terminating said trunks, trunks extending from said tandem ofiice to said second nontandem ofice, non-drop-back selectors terminating said last mentioned trunks, and multiple connections between the banks of said drop-back se-, lectors and said non-drop back selectors.

7. In a telephone system, an office comprising two groups of lines, selectors having access only to the lines of one group, other selectors having access to the lines of the other group, incoming selectors having their banks connected in multiple with the banks of the first-mentioned selectors, and trunk lines leading from said banks to selectors whose banks are connected in multiple with the banks of the second mentioned selectors.

8. In a telephone system, an ofiice comprising two groups of lines, two groups of selectors handling calls to the two groups of lines, respectively, and other selectors handling calls to one group of lines and also functioning as tandem selectors to handle calls to the other group of lines.

9. In a telephone system, an ofiice comprising two groups of lines, an order of selectors allotted to one group, selectors of the same order allotted to the other group, a second ofiice, trunks incoming from said second office,,selectors terminating said trunks and having their banks multipled with the banks of said first mentioned selectors, trunks leading from said banks, and selectors terminating said last mentioned trunks and having their banks multipled with the banks of said second mentioned selectors.

10. In a telephone system, an office comprising two groups of lines, means for establishing connections between the lines of difierent groups by a certain number of switches, means for establishing connections between lines of the same group by means of a'smaller number of switches, and means for readily changing any line from one group to another so that such line will be in the same group as a line which is frequently called.

11. In a telephone system, two groups of automatic equipment each comprising selectors and connectors, lines having called branches accessible to the connectors, calling branches for said lines having access to said selectors, and means for changing any calling branch so that particular calling lines have their calling branches in access to selectors in the same group as the connectors which are used for connecting to the lines most frequently called.

12. In a telephone system, two ofiices, two groups of tandem selectors in one office, subscribers lines accessible to said tandem selectors by .way of a varying number of additional switches, and means for insuring that a calling line in the other office will use a tandem selector which has access to the desired line by way of the minimum number of switches.

13. In a telephone system, two ofiices, a calling line in the first ofiice, :3. called line in the second ofiice which is frequently called from said first line, two routes by which calls from the first ofiice may be extended to the said called line, one route including less switches than the other, and means for routing calls from said calling line to said called line over the route which includes the smaller number of switches.

14. In a telephone system in which connections from either of two stations are extended to a third station by automatic switches operated re sponsive to identical series of digits, means for cancelling one particular digit only of the series in calls from one of said stations, and means for cancelling a difierent digit of the series in calls from the other of said stations.

15. In a telephone system in which connections from-either of two stations are extended to a third station by automatic switches set responsive toa number including a two-digit office designation, means for cancelling only the first digit of the off ce designation in calls from one of said stations, and means for cancelling only the second digit of the office designation in calls from the other of said stations.

RODNEY G. RICHARDSON.

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